Tropical storm Emily

To put things simply, dry air and modest wind shear are causing Emily many more problems than I though two days ago when the storm was still an open wave, so adjustments to forecast track/intensity are in order.

Visible and traditional IR satellite loops give the appearance of strengthening right now, with a nice, consolidated area of cold cloud tops closer to the center of Emily. Indeed, while Emily looks slightly less disheveled than yesterday with a slightly more but still not completely stacked center and more consistent convection, the low level center is still elongated and on the northwestern edge of the convection per Air Force Recon. Also, while there is some decent outflow helping to maintain this convection, arc clouds (resulting from outflow boundaries, which show dry air has intruded Emily) and an apparent lack of organized rain bands show that Emily has only slightly organized over the past day.

The most recent microwave image is grainy, but shows that Emily’s center is for the most part displaced from the deep convection and shows that there is very little banding associated with Emily’s convection, just a couple of hot towers with an expansive cirrus field evident on traditional satellite images.

A sounding from Hispaniola reveals part of the problem; there is a lot of mid level dry air. Dew point depressions (or difference between dew point and air temperatures) are well over 20 degrees Celsius through much of the sounding. This makes it very hard to maintain deep convection with a tropical cyclone or otherwise.

The San Juan sounding shows that there is also some mid level dry air (although the sounding is significantly more moist overall, due to being closer to Emily’s moisture envelope), but also reveals that while surface to 200MB shear may be light, a fast low level flow exists beneath a weak upper level flow. This is causing the surface circulation to run west faster than the mid/upper level centers, causing the cyclone’s still disheveled nature.

At the moment, it does not appear conditions around Emily will improve significantly before land interaction in about 36 hours. Water vapor imagery shows a large area of dry air due to SAL north and west of the cyclone that will only slowly mix out. While this won’t prevent intensification, shear will. The cyclone is running slightly ahead of an upper level anti-cyclone which is causing light and variable winds in the upper levels, but the above image reveals that a strong low-level easterly flow will be in place until the cyclone gets north of the Greater Antilles. Therefore, will not forecast significant intensification before land interaction.

Emily’s intensity north of interaction with either Hispaniola and/or Cuba is extremely uncertain at this point in time.

Upper level vorticity reveals a strung out upper level low to the north of Emily, with a large upper level anti-cyclone to the north. This upper level anti-cyclone is prognosticated to build south and east, causing a piece of this TUTT to cut off. The GFS shows this well:

The closed low near the Cayman Islands is our TUTT, contributing to fairly strong upper level winds north of Emily Thursday morning.

How Emily and this TUTT interact is key, because if Emily can stay east of the TUTT she will move under an upper level anti-cyclone with light vertical wind shear:

Note how by 96 hours, the GFS shows the TUTT retrograding well west into the Gulf and Emily over the Bahamas under an area of light upper level winds, which would favor intensification. The 0z Euro agrees with the same general idea, which may support a strengthening Emily once it gets north of the Greater Antilles.

Will forecast a near steady intensity at 40-45 knots through interaction with Hispaniola in about 36 hours. Will then weaken Emily to a minimal tropical storm due to interaction with tall mountains by hour 48. Will show near steady intensity through hour 72 due to potential continued land interaction and wind shear if the TUTT does not retrograde as expected. Hours 72-120 (days 4 and 5) will show a gradual re-intensification. This part of the forecast is highly uncertain, as a wide variety of solutions ranging from dissipation due to land interaction/shear to minimal damage due to land and a possibly good environment north of the Greater Antilles are both realistic solutions. For now will lean a little weaker as the combination of shear, dry air, Hispaniola/Cuba have dissipated many storms and not left many in a condition to rapidly intensify, however faster strengthening up to hurricane strength will be possible if Emily survives land interaction fairly well.

Emily is currently being steered by the southwestern edge of a large sub-tropical ridge to the W to slightly N of due W. Emily has slowed some today and will likely turn to the “right” or north as it approaches a weakness in ridging along the east coast. Models are in good agreement in this, which results in Emily either hitting head on or grazing the southern coast of Hispaniola/eastern Cuba starting Wednesday night.

The track forecast after day 3 becomes more uncertain, as the ridging may briefly build along the US east coast, making an early recurve harder to occur. Intensity after land interaction will also be highly uncertain.

The uncertainty comes from not the large scale pattern—with a negative NAO/PNA, which telleconnect to a trough along or off the east coast, but from the timing of the troughs. The east coast will see about two days between troughs at the end of this week, which will result in higher heights attempting to build back west north of Emily, keeping it from recurving out to sea early.

The 0z ECM Ensembles show the next shortwave arriving and carving off a trough along the east coast which WILL recurve Emily or what’s left of it before it affects the Mid Atlantic, but it could be a close call for Florida and the southeast coast depending on how exactly the ridging north of Emily holds up.

The 12z GFS keeps the heart of the ridging well off the US east coast and allows Emily to recurve. The 12z ECM also keeps the heart of ridging off the east coast, but shows a shallower system which is steered closer (but still off) the Florida and southeast coast by stronger surface ridging.

For my forecast for days 4-7, will recurve Emily as if the heart of ridging stays due east of the Florida coast as the 12z GFS/ECM agree on. This forecast is in fairly good agreement with the 18z ATCF consensus although is slightly west. It should be noted a weaker storm will likely track farther west closer to the Florida or the southeast coast, and that anyone living in those areas should continue to watch the progress of Emily.

Forecast track:

Forecast intensity:

12 hr (6z Wednesday): 40 knots

24 hr (18z Wednesday): 45 knots

36 hr (6z Thursday): 40 knots

48 hr (18z Thursday): 35 knots

72 hr (18z Friday): 35 knots

96hr (18z Saturday): 45 knots

120 hr (18z Sunday): 60 knots

This entry was posted in Uncategorized. Bookmark the permalink.

One Response to Tropical storm Emily

  1. Rex Ryan says:

    You made some decent points there. I looked on the internet for the issue and found most individuals will go along with with your website.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s